Reference is also made to Weinberg et al application Ser. No. 09/382,202 filed Aug. 23, 1999 and entitled MULTI-BAND, MULTI-FUNCTION, INTEGRATED TRANSCEIVER which is incorporated herein by reference.
1. Field of the Invention
This invention relates in general to wireless communication receivers. In particular, it relates to the integration of multiple signal types (CDMA, FDMA, CW, etc.), from multiple bands, with each band and signal type potentially containing multiple user channels, and a single receiver processing architecture with multiple antenna elements per band for sequentially acquiring, and simultaneously demodulating these multiple channels, utilizing jointly-optimized advanced signal processing techniques of digital beamforming, Rake multipath combining, and joint detection.
2. Description of the Prior Art
Matched Filtering
A matched-filter is typically employed in a spread-spectrum demodulator to remove the effects of PN-spreading and allow the carrier and modulating information to be recovered. The digital implementation of a matched filter can be expressed as an integrate-and-dump correlation process, which is of relatively modest computational burden during signal tracking and demodulation. However, it is computationally and/or time intensive to acquire such a signal, where many such correlations must be performed to achieve synchronization with the transmitted spreading sequence. For each potential code-phase offset to be searched (which typically number in the thousands), sufficient samples must be correlated to ensure that the integrated SNR is sufficient for detection. Performed one at a time, acquisition could easily take several minutes to achieve in typical applications.
For applications requiring rapid signal acquisition (e.g., seconds), a highly parallel matched-filter structure may be used to search many spreading code offsets simultaneously. Typically, this computationally expensive apparatus would be underutilized once acquisition is completed, during the much less demanding tracking operation. If the same parallel matched filter is also used for tracking purposes, only perhaps three of its numerous correlation branches (perhaps hundreds) are useful in this instance. Alternatively, it may be simpler to use a separate set of early, on-time, and late integrate-andidump correlators to take over once acquisition is complete; in this case, the parallel matched filter would go completely unused during tracking.
In implementations evidenced by the prior art, the matched-filtering solution has generally fallen into one of several classes:
1. Slow acquisition by sequential traversal of the search space using only the hardware required for tracking a signal; dedicated hardware per channel.
2. Rapid acquisition by parallel traversal of the search space using a dedicated parallel matched filter, which is idle or shut down when dedicated tracking hardware takes over; dedicated hardware per channel.
3. Either class 1 or 2, but multi-band and/or multi-channel, using a loosely integrated but disparate collection of individual processing resources.
Beamforming
Beamforming is a form of spatial filtering in which an array of sensor elements are utilized with appropriate signal processing to digitally implement a phased array antenna, for the purpose of shaping the antenna response over time in a space-varying manner (i.e., steering gain in some directions, and attenuation or nulls in other directions). In a radio communications system, a signal arriving at each element of an antenna array will arrive at slightly different times, due to the direction of arrival with respect to the antenna array plane (unless it has normal incidence to the plane, in which case the signal will arrive at all elements simultaneously). A phased array antenna achieves gain in a particular direction by phase-shifting, or time-shifting, the signal from each element, and then summing them in a signal combiner. By choosing the relative phasing of each element appropriately, coherence can be achieved for a particular direction of arrival (DOA), across a particular signal bandwidth.
Digital beamforming is very analogous to this, except that the signal on each antenna element is independently digitized, and the phasing/combining operation performed mathematically on the digital samples. Traditionally, digital beamforming is done on a wideband signal, prior to despreading a CDMA waveform. This forces the computationally intense beamforming to take place at a much higher sampling rate, resulting in more mathematical operations per second, and corresponding increased hardware cost (there are examples addressing this shortcoming in the prior art, such as Hanson et al., where beamforming is performed at baseband to avoid this and other issues).
Furthermore, digital beamforming is traditionally done as a separate process, independent of symbol demodulation, perhaps even as a separate product from the demodulator. In addition to the resulting inability to support advanced demodulation techniques with this architecture, the cost of the beamforming function is greater as a stand-alone function, compared to the incremental cost of adding the capability to a demodulator. The largest cost-component of beamforming is the complex multiplication of each sample for each element with the beamforming weights. When combined with the demodulator, the complex multiply can be absorbed into computation already taking place for extremely low incremental cost due to beamforming (there is, for example, an implementation of beamforming using digital direct synthesis (DDS) functions in the prior art, such as Rudish, et al.). Thus, whether stand-alone beamformers merely point in the direction of the signal of interest, or respond more adaptively to dynamic interference conditions by null-steering, they still lack the ability to be tightly coupled with potential advanced demodulation techniques.
Rake Combining
Rake combining is a method of mitigating the effects of a multipath interference dominated communications channel, as is adaptive equalization. However, in a typical equalizer, the filter time-span must correspond to the multipath delay spread, and therefore tends to be limited to very close-in multipath, spanning perhaps a few symbols. The Rake, however, exploits the properties of CDMA signals (i.e., during despreading, all other codes become uncorrelated, including copies of the desired code delayed by greater than about half a chip, and are reduced to noise across the entire spread bandwidth) that enables each multipath component (offset by more than about half a chip) to be acquired, tracked, and despread in isolation, and then coherently combined. Much like beamforming, this coherent combining results in increased effective antenna aperture and improved SNR, although using only a single antenna element. This divide-and-conquer approach allows the Rake to span an essentially arbitrary multipath delay spread, applying computational resources based linearly on the number of desired despreader branches, or xe2x80x9cFingersxe2x80x9d, desired, and not based on the delay spread itself (although acquisition time, and thus dynamic performance, is related to the actual delay spread, as this defines the limits of what must be searched).
In the prior art, Rake combining is typically employed as a dedicated function in a fixed CDMA receiver structure. Resources are designed into the receiver to perform some fixed maximum number of Rake Fingers, and those resources are tied up regardless of whether those Fingers are actually utilized or not. What is needed is a more flexible and generalized receiver architecture, which can task resources on more of a demand basis, and furthermore treat diversity information such as Rake Fingers as simply one of several diversity inputs to be jointly optimized in a common process that yields maximum advantage to each desired user signal.
What is needed is the ability to combine potential spatial processing information with other dimensions of information and diversity, both regarding the signal(s) of interest, and the interference environment. To this end, what is needed is a receiver architecture for efficiently processing spatial information (antenna elements), temporal information (coherent signal multipath components; i.e., Rake Fingers), and interference information (noise power estimates, co-channel interfering symbol soft decisions) jointly and efficiently.
The present invention applies approaches to achieve rapid acquisition in a multi-band, multi-channel signal environment, by sharing a homogeneous collection of digital processing elements. This is done, in part, by taking maximum advantage of the computational commonality between the acquisition and tracking correlation processes. Furthermore, the mismatch in computational demand between acquisition and tracking is exploited by creating a multi-channel, multi-band integrated receiver. Since only a small percentage of the computational resources are consumed by tracking an individual channel, the remaining resources may be employed to accelerate the acquisition of additional channels. As more resources become dedicated to tracking, fewer remain for acquisition; this has the effect of gradually reducing the number of parallel code offsets that can be searched, gradually increasing acquisition time. In many applications, such as a GPS receiver, this is quite acceptable, as generally additional channels beyond the first four are less urgent, and are used primarily for position refinement, and back-up signals in the event that a channel is dropped. These ideas are the subject of U.S. patent application Ser. No. 09/707,909, filed Nov. 8, 2000, entitled xe2x80x9cSequential-Acquisition, Multi-Band, Multi-Channel, Matched Filterxe2x80x9d, and are preserved as features of the present invention.
The present invention embodies various extensions to the previously disclosed invention, wherein the multi-band capability is evolved to support multiple antenna elements at a common band (as well as other bands), to support digital beamforming; the multi-channel capability is evolved to support multiple Rake Fingers on a common channel (as well as other channels); and the multi-channel demodulator capability is evolved to support computationally efficient, simultaneous processing of all bands, elements, channels, and Rake Fingers. The present invention thus forms an architectural framework capable of hosting a variety of algorithms for joint space-time optimization of individual user channels in a multipath environment, as well as multi-user (joint) detection of multiple user channels limited by co-channel interference. By considering these capabilities together, rather than as independent solutions to problems, considerable efficiencies and improvements are realized by this invention, in comparison to the prior art.
In the first aspect of the present invention, the multi-datapath receiver architecture allows independent automatic-gain control (AGC) between multiple input bands B or elements E, minimizing inter-band/element interference, and avoiding additive noise compared to schemes that combine the bands/elements into a single signal and data stream.
To accomplish this, the present invention efficiently processes multiple streams of W-bit complex sampled data (real data is easily processed as well, by adding a complex-to-read conversion to the front of the matched filter), so that multi-band or multi-element receiver signals can be kept spectrally separated. This concept, implemented using D data storage paths, supports D bands and elements when shifting at the data sampling rate (Fsamp); alternatively, the same D data storage paths can support D*k bands and elements by multiplexing the multi-band/multi-element streams and shifting the data at the higher sampling rate of k*Fsamp.
In another aspect of the present invention, the parallel acquisition correlator, or matched-filter, aids in rapid pseudo-noise (PN)-acquisition by simultaneously searching numerous possible PN-code alignments, as compared with a less compute-intensive (but more time-intensive) sequential search. Multiple channels of data may be co-resident in each band/element and sampled data stream using Code Division Multiple Access (CDMA) techniques, and multiple bands/elements and sampled data streams share the common computation hardware in the Correlator. In this way, a versatile, multi-channel receiver is realized in a hardware-efficient manner by time-sequencing the available resources to process the multiple signals, multiple antenna elements, and multiple multipath components resident in the data shift registers simultaneously.
In still another aspect of the present invention, the matched filter is organized into N xe2x80x9cSlicesxe2x80x9d of M-stages/Slice. Each Slice is composed further of D data paths supporting multiple bands B and/or antenna elements E. Each Slice can accept a code phase hand-off the from the PN-Acquisition Correlator and become a PN-tracking despreader by providing separate outputs for early, on-time, and late correlations for each element (with spacing depending on the sampling rate; typically half a chip). Slices are handed-off for tracking in the same direction as data flows, and correlation reference coefficients are shifted (for instance, left to right)-this permits shifting data to be simultaneously available for the leftmost Slices that are using the data for tracking, and rightmost Slices that are using the data for acquisition. Each Slice can choose between using and shifting the acquisition reference coefficient stream to the right, or accepting the handoff of the previous acquisition reference coefficient stream and using it to track the acquired signal.
In still another aspect of the present invention, the Acquisition correlator can integrate across all available Slices to produce a single combined output, or the individual Slice integrations can be selectively output for post-processing in the case of high residual carrier offsets or high-symbol rates, where the entire N*M-stage correlator width cannot be directly combined without encountering an integration cancellation effect. Alternatively, the Acquisition correlator can be configurable to switch from coherent integration to non-coherent integration, by taking the magnitude of I and Q partial integrations within the summer tree, or Slices themselves, at a point appropriate for the signal being acquired.
In yet another aspect, the present invention embodies a Scaleable Acquisition Correlator, which when tracking a maximum of G independent channels and/or Rake Fingers, can use the remaining N-G Slices to search for new signals for fast re-acquisition of dropped signals, and for continually searching the multipath environment for Rake Fingers to track dynamic channel conditions. Initially, Slices will be allocated sequentially (for instance, from left to right), but after running for some time, with signals alternately being acquired and dropped, the Slice allocation will most likely become fragmented, resulting in inefficient use of the Acquisition Correlator. This can be resolved by implementing a de-fragmentation algorithm that swaps tracking Slices around dynamically to maximize the number of contiguous rightmost Slices, and thus optimize Acquisition.
In another aspect, the present invention contains G independent numerically-controlled oscillator (NCO)-based PN-Code Generators with almost arbitrary code rate tracking resolution (for example, better than 0.0007 Hertz for a 32-bit NCO clocked at 3 MHz). All NCOs run using a single reference clock which is the same clock that is used for all signal processing in the Matched-Filter and Demodulator. Ultra-precise tracking of PN Code phase is maintained in the G independent phase accumulators. Multi-channel NCOS can in one embodiment be efficiently implemented by sharing computational resources and implementing phase accumulation registers in RAM, for the case when the processing rate is in excess of the required NCO sampling rate. Note that while each channel and Rake Finger requires its own PN-NCO, a single NCO is shared across all elements when beamforming.
In still another aspect of the present invention, the incoming wideband element data is made available to all Slices, which allows each element to be independently despread for each channel/Rake Finger using the core matched filter structure. As a result, beamforming is easily performed at narrowband (despread) sampling and processing rates, and with improved potential precision. The present invention is an improvement over the prior art, because in addition to the raw computational savings of narrowband processing, the beamformer hardware is time-shared across multiple elements, channels, and Rake Fingers for improved computational efficiency.
In another aspect, the present invention allows the Beamforming computation to be implemented with only additional adders, due to integration with the demodulation carrier phase rotation and the AGC scaling functions.
In yet another aspect, the present invention allows an element snapshot memory to operate at narrowband sampling rates, allowing an eased implementation for any snapshot operations required.
In still another aspect, integration of the beamformer with the demodulator in the present invention allows advanced adaptive algorithms to be implemented that can be enhanced by the feedback of post-demodulation metrics such as PN-SNR/phase, carrier-SNR/phase, symbol-SNR/phase, as well as error control decoding metrics.
In still another aspect of the present invention, the integrated beamforming CDMA Rake receiver exploits both space and time diversity aspects of a multi-path environment by assigning Slices to each Rake Finger, and steering beams that individually optimize along the line-of-sight (DOA) of each multipath reflection (i.e., a potential beam for each Rake Finger).
In another aspect of the present invention, the integrated multi-channel demodulator and Rake combiner make coherent complex symbol data for each Rake Finger (potentially for multiple user channels sharing the same frequency band), as well as individual channels not being Raked, available to a single optimization process. This allows the use of advanced multi-user detection (MUD) algorithms (e.g., joint detection) to mitigate co-channel interference that has not been suppressed by beamforming.
In yet another aspect, the present invention""s Slice-based data-flow computational architecture permits dynamic, flexible allocation of resources between tracking of multiple input bands, user channels, and Rake Fingers, and acquisition resources for dropped/new channels and continuously monitoring Rake dynamics.
In another aspect, the matched-filter Slice architecture of the present invention contains PN-tracking integrators (i.e., early, on-time, late) for each beamforming element. Furthermore, after all elements are weighted and combined, the demodulator architecture uses the combined early/on-time/late integrations to maintain a single PN-tracking loop for each beamforming channel, or Rake Finger.
In another aspect, the present invention allows each beamforming channel, or Rake Finger, to combine data from all elements and form a composite carrier and symbol discriminator that allows all elements of that channel to be tracked with a single carrier loop, and a single symbol loop.
In still another aspect, the present invention""s multi-channel architecture allows continuous on-line element calibration capability to take place. Furthermore, calibration can be performed independently on each user channel, and each Rake Finger, closing the calibration loops individually to remove essentially all bias terms.